Methods of producing boards of lignocellulosic material are well known and have significant practical applications. Such manufacturing methods include the following main steps: breaking up of the raw material into particles having a suitable size and/or fibers, drying to a predetermined moisture ratio, and gluing the material before or after the drying step, forming the glued material into a mat which may be constructed of several layers, possibly cold pre-compressing, preheating, water spraying of surfaces, etc., a heat compressing under pressure and heat in a stroke compressor or a continuous compressor until the board is finished.
During conventional heat compressing, the compressed material is primarily heated by using heat coils from adjacent heating plates or steel bands. These have a temperature of 150-200.degree. Celsius depending on the type of product that is being compressed, the type of glue used, desired capacity, etc. In this way, the moisture in the material closest to the heat sources is evaporated so that a dry layer is developed in this area and the steam front gradually moves towards the center of the board from each side as compression continues. When the dry layer has been developed this means that the temperature in this layer is at least 100.degree. Celsius, which initiates the curing of conventional glues. When the steam front has reached the center, the temperature at the center has reached at least 100.degree. Celsius, and the board even starts to cure at its center so that the compression can be stopped within a couple of seconds. This applies to situations when conventional urea formaldehyde glue (UF) and similar glues are used, such as melamine fortified (MUF) glue. When other glues, having a higher curing temperature, are used, then a higher temperature and a greater steam pressure is required in the board before any curing can start.
To achieve the desired density, a compressor must apply a high surface pressure at a high temperature. This is not a problem for non-continuous compression in a so-called stroke compressor but such compressors have other drawbacks, such as worse thickness tolerances, etc. When continuous compressors are used, the requirement for high surface pressures and high temperatures at the same time have led to expensive high precision solutions with regard to the roller felt between the steel band and the heating plate positioned therebelow. The method of providing heat to the board by means of heat coils makes the heating relatively time consuming, which results in long compression lengths (large compression surfaces).
The heating can also be achieved by delivering steam to the mat to be compressed. In this way, the heating time is drastically shortened and, in addition, the resistance of the material to compression is drastically reduced when steam is introduced so that less compression forces and smaller compression surfaces are required. An injection box may be used to inject steam into the material mat which, however, has certain drawbacks. To avoid these drawbacks, compression rollers have been developed that are perforated and function as a steam delivery member. Such apparatus is disclosed in Swedish Patent No. 502,810.
The use of steam injection for heating the material is well known in the industry. For example, reference is made to European Patent No. 383,572; U.S. Pat. No. 2,480,851; British Patent No. 999,696; German Patent No. 2,058,820; German Patent Application Nos. 36 40 682 and 40 09 883; and Australian Patent No. 57390/86, which show different examples of how steam is injected during continuous processes to produce fiber boards.
According to the method of German Patent Application No. 36 40 682, the steam injection is applied during a pre-compression stage. Immediately after pre-compression, the material mat is passed near a steam box or a similar device where it is exposed to a steam flow to such an extent that the curing temperature of the binders are not exceeded, which normally means a temperature of between about 65 and 90.degree. Celsius. The material is then compressed to a completed form while being exposed to additional heat so that it cures.
Because the steam in this method is injected after the actual pre-compression step and is mainly a preparation for subsequent treatment steps, it does not affect the condition of the material mat in the pre-compression step.
According to the method of German Patent Application No. 20 58 820, the steam is introduced during the actual pre-compression step. This can result in the temperature of the fiber mat being increased such that the glue or binders start to cure, which with conventional glues occurs at a temperature exceeding 100.degree. Celsius. This creates problems for performing the subsequent treatment step.
The object of the present invention is to achieve pre-compression with steam delivery in such a way that the subsequent steps are not made more difficult to perform.